Refrigeration process with purge and recovery of refrigerant

ABSTRACT

A refrigeration process including a refrigeration cycle, and refrigerant purge and recovery operations is disclosed. The refrigeration cycle may be a vapor compression cycle or an absorption cycle, for example. A purge stream is withdrawn from the refrigeration cycle and subjected to treatment by means of a membrane separation unit. The purge-stream treatment operation produces an essentially pure refrigerant stream, suitable for return to the refrigeration cycle, and an air stream, clean enough for direct discharge to the atmosphere. The process is applicable to most refrigerants, but is particularly useful in minimizing atmospheric emissions of chlorofluorocarbons, such as CFC-11 and CFC-12.

FIELD OF THE INVENTION

This invention relates to a refrigeration process. More particularly,the invention relates to a refrigeration process in whichrefrigerant/air mixtures are purged from the refrigeration cycle andtreated by means of a membrane separation process to recover refrigerantand to reduce atmospheric pollution.

BACKGROUND OF THE INVENTION

Refrigeration is the use of mechanical or heat-activated machinery forcooling purposes. Refrigeration is commonly accomplished in a reverseCarnot cycle, by using as refrigerant a fluid that evaporates andcondenses at suitable pressures and temperatures to enable practicalequipment to be manufactured. In a vapor compression refrigerationcycle, the vapor is typically compressed, then condensed by chillingwith air or water, then expanded to a low pressure and correspondinglylow temperature through an expansion valve. Subsequent evaporation ofthe refrigerant provides the cooling action. In an absorptionrefrigeration cycle, cooling is also achieved by expansion of ahigh-pressure vapor into a low-pressure region. The resultinglow-pressure vapor is absorbed into water, then separated from the waterat high pressure in a stripper.

Many fluids that can serve as refrigerants under appropriate conditionsare known. Refrigerants are generally grouped into three classes,depending on their toxicity and flammability. Group 3 refrigerants arehighly toxic or flammable, and are therefore used only in specialcircumstances, such as where the refrigerant is available on-site as aprocess or product chemical, and the existing hazard is not exacerbatedby the use. Such refrigerants include hydrocarbons such as methane,propane and butane. Group 2 refrigerants are slightly toxic orflammable, and include ammonia, which is still used widely, as well assulfur dioxide. Group 1 refrigerants are non-toxic and non-flammable,and are, therefore, the most widely used over a broad spectrum ofrefrigeration needs. Mosts of the Group 1 refrigerants are halogenatedhydrocarbons, containing one or more chlorine, fluorine or bromine atomsin their structures. For example, industrial refrigerators use vastquantities of CFC-12 and other chlorofluorocarbons (CFCs), which,although they are non-toxic and non-flammable, are now recognized tohave a disastrous environmental impact.

Refrigeration can be carried out either as a closed-cycle or open-cycleprocess. Open-cycle operation is mostly used in the chemical processindustry, where advantage is taken of the presence in the chemicalprocess of a product that can also serve as refrigerant. For example,natural gas liquids removed by cooling and compressing raw natural gasmay be expanded in a refrigeration cycle to further lower thetemperature of the raw gas, thereby recovering more of the heavierhydrocarbons. Ammonia synthesis plants use the product stream torefrigerate ammonia storage tanks.

For most other industrial purposes, closed-cycle refrigerators are used.The refrigerant is contained in an essentially closed loop, where itcycles round from high-pressure vapor to high-pressure liquid tolow-pressure liquid to low-pressure vapor. The low-pressure, evaporatingportion of the system may be at atmospheric pressure or may be belowatmospheric pressure, depending on the thermodynamic properties of therefrigerant and the cooling temperature. For practical reasons,refrigeration systems using CFC refrigerants are frequently operatedwith the evaporating pressure as low as 2-5 psia.

Because a large portion of the refrigeration system is atsub-atmospheric pressure, air leaks into the system on the low pressureside. Air leaks are almost unavoidable in large industrialrefrigerators; thus air contaminated with refrigerant vapor must beperiodically purged from the system. In conventional purge systems, agas stream, containing refrigerant and air, is withdrawn from thehigh-pressure side of the cycle. To reduce the refrigerant loss, thestream is maintained at the high purge pressure and then cooled,typically down to as low as -50° F. or below. The low-temperaturerefrigerant can conveniently be used to effect the cooling. Under theseconditions, the bulk of the refrigerant contained in the stream iscondensed and passed back to the refrigerator. The remainder is ventedto the atmosphere. The frequency and thoroughness with which the purgingoperation is carried out is dictated by energy and economicconsiderations. If the air content within the loop is allowed to buildup over a prolonged period, the partial pressure of the air in thesystem may become substantial. As a result, the total compressorpressure required to maintain the refrigerant partial pressure at anadequate level becomes higher and higher, with a corresponding increasein energy consumption and costs.

The air content of the refrigerator can be kept at a constant low levelby continuous purging. Cooling the purge gas typically enables as muchas 90% or more of the refrigerant to be recovered from the purge streamby condensation. Nevertheless the air that is vented to the environmentmay contain as much as 15% refrigerant. Running the purge-gas treatmentcondenser at pressure and temperature conditions where essentially norefrigerant is lost imposes an excessively heavy load on the condenser,consumes excessive energy, and becomes impractical economically. Theneed to drastically control or eliminate CFC emissions to the atmospherehas been recognized throughout the world and is the subject ofincreasingly stringent regulatory laws. CFC refrigerants, besides theirenvironmental unacceptability, are becoming increasingly expensive.Refrigerator discharges represent a serious environmental problem andwaste of resources. A 10 scfm condenser vent stream containing 5% ormore CFC is typical of many that are found throughout the food andpharmaceutical industries, for example. Such a discharge corresponds toa CFC loss of 0.16 lb/min, or approximately 80,000 lb/yr. Whenmultiplied by the many hundred industrial refrigeration plants in usenationwide, this rate of loss represents a large source of CFC pollutionand waste resources. Thus there is an urgent need to improverefrigeration technology to drastically reduce or eliminate CFCdischarges. Similar, if less critical, concerns apply to otherrefrigerants. Because of the adverse effect on the operation of therefrigeration cycle, there is also a need for improved methods ofkeeping the air content of the cycle as low as possible.

Attempts have been made to monitor and/or treat purge streams fromrefrigeration operations by various means besides condensation. Forexample, U.S. Pat. No. 4,485,289 to Lofland describes a distillationprocess for recovering CFCs from refrigerator purge streams. U.S. Pat.No. 4,531,375 to Zinsmeyer describes a refrigeration system includingmeans for monitoring a refrigerator purge system and correcting excessdischarge of purge gases. U.S. Pat. No. 4,484,453 to Niess describes amethod for controlling non-condensable gases at a predeterminedconcentration in an ammonia refrigerator by sensing the temperature atwhich the ammonia condenses.

Separation of gas or vapor mixtures by means of permselective membraneshas been known to be possible for many years, and membrane-based gasseparation systems are emerging to challenge conventional separationstechnology in a number of areas. That membranes have the potential toseparate organic or inorganic vapors from air is known. For example,U.S. Pat. No. 4,553,983, commonly owned with the present invention,describes a process for separating airstreams containing lowconcentrations of organic vapor (2% or less) from air, using highlyorganic-selective membranes. U.S. Pat. No. 3,903,694 to Aine describes aconcentration driven membrane process for recycling unburnt hydrocarbonsin an engine exhaust. U.S. Pat. No. 2,617,493 to Jones describesseparation of nitrogen from concentrated hydrocarbon feedstreams.Pending patent application Ser. No. 327,860, now U.S. Pat. No.4,906,256, commonly owned with the present invention, describes amembrane separation process for treating air or other gas streamscontaining fluorinated hydrocarbons, such as CFCs.

SUMMARY OF THE INVENTION

The invention is an improved refrigeration process, involving thecombination of a refrigeration cycle, a purge operation to remove air orother non-condensable gases from the refrigerator, and treatment of thepurged gas by a membrane separation system to recover the refrigerant.

The refrigeration cycle is preferably a closed-cycle operation in whicha refrigerant is brought to a low temperature, for example, either byvapor compression or absorption. The refrigeration cycle may take theform of (a) a simple cycle, in which a single refrigerant circulating ina single cycle is used, (b) a compound cycle in which more than onecompression/expansion cycle is used, but a common refrigerant circulatesthroughout, or (c) a cascade, in which a series of separaterefrigeration cycles are used to achieve successively lowertemperatures.

Any of the refrigerants known in the art may be used, includinginorganic compounds such as ammonia, sulfur dioxide or carbon dioxide,saturated and unsaturated hydrocarbons such as propane, butane, ethyleneor propylene, and halogenated hydrocarbons, such as many of thechlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Thepurpose of the purge operation is to remove air or any othernon-condensable gases that have entered the refrigeration system. It isdesirable to maintain the amount of air circulating with the refrigerantat a very low level, because the presence of air in the refrigerantvapor means that the compressor has to operate at higher pressures thanwould otherwise be necessary. As the level of air in the system buildsup, the system becomes more and more inefficient. The purge operationinvolves withdrawing a portion of the refrigerant vapor, eithercontinuously or periodically. For example, the vapor may be withdrawnthrough a pressure-actuated valve, connected in the high-pressure vaporportion of the cycle, that opens automatically when the pressure withinthe system exceeds a certain value. The purged vapor may contain fromonly very small amounts of non-condensable gas, such as less than 1%, upto a substantial percentage, say 5%, 10%, 15%, or above.

The treatment of the purge stream is designed to recover as much of therefrigerant as possible, and to leave a residual air stream that isclean enough for discharge to the atmosphere without adverseenvironmental effects. The purge-stream treatment operation involvesseparating the refrigerant and air by running the purge gas streamacross a membrane that is selectively permeable to the refrigerant. Therefrigerant is therefore concentrated in the stream permeating themembrane; the residue, non-permeating, stream is correspondinglydepleted in refrigerant. The driving force for permeation across themembrane is the pressure difference between the feed and permeate sides.If the purge stream is withdrawn from the high-pressure portion of thecycle, as is normally done, no additional driving force for membranepermeation need be provided. The membrane separation process produces apermeate stream enriched in the refrigerant compared with the feed and aresidue stream depleted in the refrigerant.

The efficiency of the process, in terms of the relative proportions ofrefrigerant and air in the feed, permeate and residue streams, will bedetermined by a number of factors, including the pressure difference,the selectivity of the membrane, the proportion of the feed thatpermeates the membrane, and the membrane thickness. The presentinvention is applicable to feedstreams with a broad range of refrigerantconcentrations. Effective membrane separation is possible, even when themembrane selectivity is modest. In one possible embodiment, the processproduces a permeate stream from which the pure liquid refrigerant can berecovered by cooling and/or compressing the permeate stream. The liquidrefrigerant might then be returned to the refrigeration cycle in theliquid portion of the cycle. As another option, it may be possible toreturn the permeate vapor directly to the refrigeration cycle on thelow-pressure vapor side.

The membrane separation process may be configured in many possible ways,and may include a single membrane stage or an array of two or more unitsin series or cascade arrangements. Eighty to 99% or above removal of therefrigerant content of the feed to the membrane system can be achievedwith an appropriately designed membrane separation process, leaving aresidue stream containing only traces of the refrigerant. The permeatestream is typically concentrated 5- to 100-fold compared with thefeedstream.

The discussion above describes embodiments of the invention in which themembrane used to separate the refrigerant from air is preferentiallypermeable to the refrigerant. Embodiments of the invention in which themembrane is selectively permeable to air are also possible. In this casethe non-permeating, or residue, stream is enriched in the refrigerant.The particular advantage of this method of operation is that, dependingon the selectivity of the membrane, the nature of the refrigerant, andoperating parameters, it may be possible to maintain a substantiallylower air level within the refrigerator than can be economicallyachieved using the refrigerant-selective membrane options. In this casealso, the membrane separation step may be configured as a single ormultiple stage operation.

If refrigerant-selective membranes are used, a preferred embodiment ofthe invention incorporates a purge-gas treatment step in which the purgegas is passed through a condenser prior to entering the membraneseparation unit. Purge-gas streams have previously been treated bycondensation and many industrial refrigerators are already fitted withcondensers for this purpose. The purge gas withdrawn from therefrigerator is normally at high pressure, for example, around 100 psi,so simply cooling the purge gas will cause a fraction of the refrigerantto condense out. Conventional condensation units attached torefrigerators may remove up to about 90% of the refrigerant that passesthrough them. If the condenser is followed by a membrane separationunit, the requirement for a high degree of removal by the condenser maybe eased. The condenser may be operated at a less cold temperature,thereby saving energy and costs, and yet achieving essentially completerecovery of the refrigerant. The membrane system can typically remove 90or 95% of the refrigerant that reaches it from the condenser. Thus thecombination of the condenser and the membrane separator will achieve amuch higher degree of refrigerant recovery than could be achieved by thecondenser alone. For example, suppose the condensation step removes only50% of the refrigerant. If the condensation step is followed by amembrane separation step that can remove 80% of the refrigerant reachingit, then the total removal obtained by the process is 90%. If thecondensation step removes 80%, and is followed by a membrane separationstep that also removes 80%, then the total removal obtained by theprocess is 96%. If the condensation step removes 80% and the membraneseparation step 90%, the total removal is 98%.

The process of the invention exhibits a number of advantages overconventional refrigeration processes. Membrane separation systems arecharacterized by low energy consumption compared with other separationtechniques. The driving force for permeation through the membrane isprovided by a pressure difference between the feed and permeate sides ofthe membrane. In the present process, the purge gas from therefrigerator is already at a high pressure compared with atmosphericpressure, so the membrane separation can be achieved in some embodimentswithout supplying any additional energy at all. The value of theadditional refrigerant that is recovered may be substantial,particularly in the case of CFCs and HCFCs. Thus the process of theinvention can provide a much improved recovery rate for the refrigerant,for example from 80% to 95%, or from 90% to >99%, coupled with a netreduction in the operating cost. Another significant advantage, againparticularly important for CFCs and HCFCs, is that the amount ofrefrigerant discharged to the environment as vent gas is reduced by 90%or more. By providing a more efficient purge cycle, the process of thepresent invention also reduces the energy demand on the compressor inthe refrigeration cycle, because it becomes easier and cost effective tomaintain a lower circulating air content.

It is an object of the invention to provide a refrigeration process inwhich emissions of refrigerant to the atmosphere are eliminated orminimized.

It is an object of the invention to provide an improved method oftreating refrigerator purge gases.

It is an object of the invention to provide an energy-savingrefrigeration process.

It is an object of the invention to separate refrigerants from air.

It is an object of the invention to reduce the load on a compressor usedin a refrigeration cycle.

It is an object of the invention to reduce the load on a condenser usedto recover purged refrigerant.

Other objects and advantages of the invention will be apparent from thedescription of the invention to those of ordinary skill in the art.

It is to be understood that the above summary and the following detaileddescription are intended to explain and illustrate the invention withoutrestricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing an embodiment of the invention, employinga condenser and a refrigerant-selective membrane.

FIG. 2 is a schematic showing an embodiment of the invention wherein themembrane permeate vapor is returned directly to the refrigeration cycle.

FIG. 3 is a schematic showing an embodiment of the invention wherein themembrane permeate stream is returned to the condenser.

FIG. 4 is a schematic showing an embodiment of the inventionincorporating a nitrogen-selective membrane.

FIG. 5 is a schematic showing an embodiment of the inventionincorporating a condenser followed by a nitrogen-selective membrane.

FIGS. 6 and 7 show optional configurations for the membrane system. FIG.6 is a schematic of a two-stage membrane system. FIG. 7 is a schematicdiagram of a two-step membrane system.

FIG. 8 is a graph showing the relationship between feed and permeateconcentrations of CFC-11 at low CFC feed concentrations.

FIG. 9 is a graph showing the relationship between CFC-11 flux and feedconcentration at low CFC feed concentrations.

FIG. 10 is a graph showing the relationship between feed and permeateconcentrations of CFC-11 at CFC feed concentrations up to about 35 vol%.

FIG. 11 is a graph showing the relationship between membrane selectivityand feed concentration of CFC-11 at CFC feed concentrations up to about35 vol%.

FIG. 12 is a graph showing the relationship between feed and permeateconcentrations of CFC-113 at low CFC feed concentrations.

FIG. 13 is a graph showing the relationship between feed and permeateconcentrations of CFC-113 at CFC feed concentrations up to about 6 vol%.

FIG. 14 is a graph showing the relationship between feed and permeateconcentrations of HCFC-123 at low CFC feed concentrations.

FIG. 15 is a graph showing the relationship between feed and permeateconcentrations of HCFC-123 at CFC feed concentrations up to about 8vol%.

FIG. 16 is a graph showing the relationship between feed and permeateconcentrations of HCFC-142b at low CFC feed concentrations.

FIG. 17 is a graph showing the relationship between feed and permeateconcentrations of CFC-114 at CFC feed concentrations up to about 25vol%.

DETAILED DESCRIPTION OF THE INVENTION

The term vapor as used herein refers to organic or inorganic chemicalcompounds in the gaseous phase below their critical temperatures.

The term CFC as used herein refers to hydrocarbons containing at leastone fluorine atom and one chlorine atom.

The term HCFC as used herein refers hydrocarbons containing at least onefluorine atom, one chlorine atom and one hydrogen atom.

The term hydrocarbon as used herein means saturated or unsaturatedhydrocarbons.

The term permselective as use herein refers to polymers, or membranesmade from those polymers, that exhibit selective permeation for at leastone gas or vapor in a mixture over the other components of the mixture,enabling a measure of separation between the components to be achieved.

The term permeability of a polymer film means the rate at which a gas orvapor passes through a unit cross section of that film of unit thicknessunder a unit driving force.

The term selectivity means the ratio of the permeabilities of two gasesor vapors, the permeabilities being determined with a mixture of gasesor vapors at concentrations and under operating conditionsrepresentative of an actual membrane separation system.

The term air-selective means selective for nitrogen and selective foroxygen over the refrigerant.

The term multilayer as used herein means comprising a support membraneand one or more coating layers.

The term residue stream means that portion of the feedstream that doesnot pass through the membrane.

The term permeate stream means that portion of the feedstream thatpasses through the membrane.

The term stage-cut as used herein means the ratio of the membranepermeate volume flow to the membrane feed volume flow.

The term membrane unit as used herein means one or more membrane modulesarranged in parallel, so that a portion of the incoming stream passesthrough each one.

The term series arrangement means an arrangement of membrane modules orunits connected together such that the residue stream from one module orunit becomes the feedstream for the next.

The term cascade arrangement means an arrangement of membrane modules orunits connected together such that the permeate stream from one moduleor unit becomes the feedstream for the next.

The term membrane array means a set of one or more individual membranemodules or membrane units connected in a series arrangement, a cascadearrangement, or mixtures or combinations of these.

The term product residue stream means the residue stream exiting amembrane array when the membrane separation process is complete. Thisstream may be derived from one membrane unit, or may be the pooledresidue streams from several membrane units.

The term product permeate stream means the permeate stream exiting amembrane array when the membrane separation process is complete. Thisstream may be derived from one membrane unit, or may be the pooledpermeate streams from several membrane units.

All percentages cited herein are by volume unless specifically statedotherwise.

The refrigeration process of the invention is a combination of arefrigeration cycle, withdrawing a purge stream, and treating the purgestream. The refrigeration cycle of the invention is a conventionalmechanical cycle of either the vapor or the absorption type. The vaporcycle includes as basic elements a compressor, a condenser, an expansionvalve and an evaporator, in that order. The compressor raises thepressure of the refrigerant vapor so that its saturation temperature isabove the temperature of the coolant in the condenser. A transfer ofheat from the refrigerant vapor to the coolant takes place, and therefrigerant condenses. The condensed liquid passes through the expansionvalve to a low-pressure region of the cycle, where its saturationtemperature is below the temperature of the material to be refrigerated.A transfer of heat takes place to the refrigerant, causing therefrigerant to evaporate. The low pressure vapor is removed by thecompressor and the cycle continues. The compressors and condensers usedin the refrigeration cycle may take any of the common forms known in theart. For example, the compressor may be of the reciprocating orcentrifugal types. Multiple compressors arranged in series or parallelmay be used. The condenser may conveniently be water cooled, such as abasic shell-and-tube condenser, or air cooled, in which case air may beblown over the condenser by propeller fans. The evaporator may also beof the shell-and-tube design, where the exchange surfaces are eitherflooded or sprayed with refrigerant liquid.

In an absorption cycle, the refrigerant, usually ammonia, is alternatelyabsorbed into water, then stripped from the water at high pressure. Thecycle includes the stripper, a condenser, an expansion valve, anevaporator and an absorber. Ammonia at high pressure is withdrawn fromthe top of the stripper, condensed and passed through an expansion valveto form a low-pressure liquid. As in the vapor cycle, the liquid thenpasses through the evaporator. The resulting low-pressure vapor isabsorbed into water in a conventional absorber, and the resulting liquidis pumped back to the stripper.

The construction and operation of refrigeration cycles is well known inthe art. For more details, the four-volume handbook published by theAmerican Society of Heating, Refrigerating and Ventilating Engineers,Publications Dept., Atlanta, Ga., provides comprehensive information onrefrigerator design considerations and equipment.

The refrigeration cycle used in the process of the invention may besimple, complex or cascade.

Any refrigerant may be used in the process of the invention.Representative refrigerants include, from Group 1, carbon dioxide,CFC-11, CFC-12, CFC-13, CFC-22, CFC-23, CFC-113 and CFC-114, HCHF-123,HCFC-142b and HCFC-134a from Group 2, carbon tetrachloride, ammonia andsulfur dioxide, and from Group 3, methane, ethane, propane, butane,isobutane, ethylene and propylene.

The purge stream, containing the refrigerant and air that has enteredthe refrigeration cycle, is withdrawn from the refrigeration cycleeither continuously or periodically. The rate of withdrawal can bechosen, depending on the amount of air leakage into the system and thelevel to which the air content within the cycle is to be reduced or atwhich it is to be maintained. It is preferable to withdraw the purgecontinuously, or for lengthy periods, if possible, so that the membraneunit can also operate without the need for frequent shut-down andstart-up. A valve is used to control the refrigerant withdrawal. Thevalve may be manually operated, or may be automatically opened atcertain time intervals or at a chosen pressure, for example. The purgestream contains refrigerant and air, preferably, although notnecessarily, with air as the minor component of the stream. Typicallythe air content of the purge stream may range from less than 1% to 50%.Preferably, the purge stream is withdrawn from the high-pressure vaporsegment of the refrigeration cycle. In a cascade cycle, separate purgestreams may be withdrawn from each stage.

A membrane system is used to treat the purge stream. The permeability ofa gas or vapor through a membrane is a product of the diffusioncoefficient, D, and the Henry's law sorption coefficient, k. D is ameasure of the permeant's mobility in the polymer; k is a measure of thepermeant's sorption into the polymer, and depends in part on thecondensability of the vapor. The diffusion coefficient tends to decreaseas the molecular size of the permeate increases, because large moleculesinteract with more segments of the polymer chains and are thus lessmobile. Depending on the nature of the polymer, either the diffusion orthe sorption component of the permeability may dominate. In rigid,glassy polymer materials, diffusion is usually the predominant factorcontrolling permeation, so glassy membranes tend to permeate nitrogen oroxygen, for example, faster than the larger organic molecules. Forembodiments of the invention requiring an air-selective membrane,therefore, membranes made from glassy polymers are preferred.

In elastomeric membrane materials, the effect of the sorptioncoefficient can be dominant, so that the condensable refrigerant willpermeate the membrane much faster than nitrogen or oxygen. Forrefrigerant-selective embodiments of the invention, therefore,elastomeric membrane materials are preferred. Hydrophobic elastomers arepreferred for hydrophobic refrigerants; for hydrophilic refrigerants,such as ammonia, carbon dioxide and sulfur dioxide, more hydrophilicmaterials may be more suitable.

In the process of the present invention, the purge stream from therefrigerator cycle, which may optionally have been subjected tocondensation to recover a portion of the refrigerant, and which containsrefrigerant vapor and air is passed across a thin, permselectivemembrane. The permselective membrane forms a barrier that is relativelypermeable to one component of the stream, but relatively impermeable tothe other. The membrane may take the form of a homogeneous membrane, amembrane incorporating a gel or liquid layer, or any other form known inthe art.

To achieve a high flux of the permeating components, the permselectivemembrane should be made as thin as possible. Preferred embodiments ofthe invention involve the use of a composite membrane comprising amicroporous support, onto which the permselective layer is deposited asan ultrathin coating. The microporous support membrane should have aflow resistance that is very small compared to the permselective layer.A preferred support membrane is an asymmetric membrane, consisting of arelatively open, porous substrate with a thin, dense, finely porous skinlayer. Suitable polymers for making asymmetric support includepolysulfones, polyimides, polyamides and polyvinylidene fluoride.Asymmetric polysulfone and polyimide membranes available commerciallyfor ultrafiltration applications are also suitable as supports.Isotropic support membranes, such as microporous polypropylene orpolytetrafluoroethylene may also be used in some cases. The thickness ofthe support membrane is not critical, since its permeability is highcompared to that of the permselective layer. However the thickness wouldnormally be in the range 100 to 300 microns, with about 150 micronsbeing the preferred value.

Optionally, the support membrane may be reinforced by casting it on afabric or paper web, made from polyester or the like. The multilayermembrane then comprises the web, the microporous membrane, and theultrathin permselective membrane.

The permselective membrane is deposited on the dense skin of the supportmembrane, for example by dip coating. The dip coating method isdescribed, for example, in U.S. Pat. No. 4,243,701 to Riley et al. In atypical dip-coating process, the support membrane from a feed roll ispassed through a coating station, then to a drying oven, and is thenwound onto a product roll. The coating station may be a tank containinga dilute polymer or prepolymer solution, in which a coating typically 50to 100 microns thick is deposited on the support. Assuming a 1%concentration of polymer in the solution, after evaporation a film 0.5to 1 micron thick is left on the support.

Alternatively, the permselective membrane may be cast by spreading athin film of the polymer solution on the surface of a water bath. Afterevaporation of the solvent, the permselective layer may be picked uponto the microporous support. This method is more difficult in practice,but may be useful if the desired support is attacked by the solvent usedto dissolve the permselective material.

The thickness of the permselective layer should normally be in the range0.1 to 20 microns, preferably 5 microns or less, and more preferably 0.1to 2 micron.

Preferred polymers for use as refrigerant-selective membranes includerubbery non-crystalline polymers, i.e. those that have a glasstransition temperature below the normal operating temperature of thesystem. Thermoplastic elastomers are also useful. These polymers combinehard and soft segments or domains in the polymer structure. Provided thesoft segments are rubbery at the temperature and operating conditions ofthe invention, polymers of this type could make suitable membranes foruse in the invention. Polymers that may be used include, but are notlimited to, nitrile rubber, neoprene, polydimethylsiloxane (siliconerubber), chlorosulfonated polyethylene, polysilicone-carbonatecopolymers, fluoroelastomers, plasticized polyvinylchloride,polyurethane, cis-polybutadiene, cis-polyisoprene, poly(butene-1),polystyrene-butadiene copolymers, styrene/butadiene/styrene blockcopolymers, styrene/ethylene/butylene block copolymers, thermoplasticpolyolefin elastomers, and block copolymers of polyethers, polyamidesand polyesters. To maximize the flux of permeating components, thepermselective layer should be made as thin as possible. However, thepermselective layer must also be free of pinholes or other defects thatcould destroy the selectivity of the membrane by permitting bulkflow-through of gases. In the context of the invention, a particularlypreferred rubber is silicone rubber. Silicone rubber solutions can wet afinely microporous support and leave a uniform, defect-free coatingafter solvent evaporation, so the preferred membrane is one in which thepermselective coating is deposited directly on the microporous support.However optional embodiments that include additional sealing orprotective layers above or below the permselective layer are alsointended to be encompassed by the invention.

Preferred polymers for use as air-selective membranes include glassymaterials such as polysulfones, polyimides, polyamides, polyphenyleneoxide, polycarbonates, ethylcellulose or cellulose acetate. Glassymaterials are more difficult than elastomers to form into thin-filmcomposite membranes. Preferred embodiments of the process employingair-selective membranes use asymmetric glassy membranes in which thethin, dense skin serves as the permselective layer. Such membranes areknown in the gas-separation art, and may be prepared, for example, byvarious modifications of the Loeb-Sourirajan process. This processinvolves preparing a solution of the polymer in a suitable solvent,casting a thin film and then immersing the film in a precipitation bath.The resulting membrane has an asymmetric structure graded from openlymicroporous on the support surface to non-porous or very finelymicroporous on the skin side. Such gas-separation membranes arefrequently overcoated with a sealing layer on the skin side, to preventbulk flow of gases through pores or other defects. The preparation andproperties of asymmetric gas-separation membranes are described, forexample, in U.S. Pat. No. 4,230,463 to Henis and Tripodi, or U.S. Pat.No. 4,840,646 to Dow Chemical.

The permselective membranes used in the present invention shouldpreferably have a selectivity for refrigerant/nitrogen ornitrogen/refrigerant of at least 5, and more preferably at least 10, andmost preferably at least 20. Table 1 lists experimentally measuredselectivities for a number of common refrigerants. In each case, themembrane used was a thin-film composite membrane with a silicone rubberpermselective layer. The measurements were made at 20° C. As a generalrule, lowering the temperature will increase the selectivity and viceversa. For refrigerants such as CHF₃, where the membrane selectivity isonly 4 at room temperature, a much better membrane performance would beobtained by performing the membrane separation operation at lowtemperature. As shown in the table, the membrane selectivity increasesto 14 at -39° C.

                  TABLE 1                                                         ______________________________________                                        Membrane Selectivity for Refrigerant Over Nitrogen for                        Common Refrigerants, measured with a silicone rubber membrane                 at 20° C.                                                                                   Boiling point                                                                            Selectivity                                   Refrigerant  Group   (°C.)                                                                             (at 20° C.)                            ______________________________________                                        Inorganic Compounds                                                           Ammonia      2       -33        20                                            Sulfur dioxide                                                                             2       -10        50                                            Carbon dioxide                                                                             1       -18        11                                            Hydrocarbons                                                                  Methane      3       -161        3                                            Ethane       3       -89        10                                            Propane      3       -42        20-40                                         Butane       3         0         70-100                                       Isobutane            -12         70-100                                       Ethylene     3       -104       2-4 (est)                                     Propylene    3       -48        15-20                                         Chlorinated                                                                   Hydrocarbons                                                                  CCl.sub.4    2        77        100-200                                       CCl.sub.3 F  1        24        30-50                                         CCl.sub.2 F.sub.2                                                                          1       -30         6                                            CClF.sub.3   1       -81        0.6                                           CHClF.sub.2  1       -41        15                                            CHF.sub.3    1       -82        4 (14 at -39° C.)                      CCl.sub.2 FCClF.sub.2                                                                      1        48        20                                            CClF.sub.2 CClF.sub.2                                                                      1         4         9-11                                         C.sub.2 HCl.sub.2 F.sub.3                                                                  1                  25                                            C.sub.2 H.sub. 3 ClF.sub.2                                                                 1                  13-15                                         ______________________________________                                    

The form in which the membranes are used in the invention is notcritical. They may be used, for example, as flat sheets or discs, coatedhollow fibers, or spiral-wound modules, all forms that are known in theart. Spiral-wound modules are a preferred choice. References that teachthe preparation of spiral-wound modules are S. S. Kreman, "Technologyand Engineering of ROGA Spiral Wound Reverse Osmosis Membrane Modules",in Reverse Osmosis and Synthetic Membranes, S. Sourirajan (Ed.),National Research Council of Canada, Ottawa, 1977; and U.S. Pat. No.4,553,983, column 10, lines 40-60. Alternatively the membranes may beconfigured as microporous hollow fibers coated with the permselectivepolymer material and then potted into a module.

The flux of a gas or vapor through a polymer membrane is proportional tothe pressure difference of that gas or vapor across the membrane. Toachieve high fluxes of the permeating components, it is desirable notonly to make the permselective membrane very thin, but also to operatethe system with a substantial pressure drop across the membrane. Thepurge gas stream is withdrawn from the high-pressure vapor segment ofthe refrigeration cycle. The purge gas stream is therefore normally at apressure substantially above atmospheric, and may be at a pressure ashigh as 100 psia or 200 psia. Consequently an adequate driving force formany embodiments of the invention may be provided by keeping thepermeate side of the membrane at atmospheric pressure and using the highpressure inherently available in the refrigeration cycle. Theperformance of the membrane system depends not only on the membraneselectivity and the pressure drop across the membrane, but also on theratio feed pressure: permeate pressure. It can be shown theoreticallythat, even for an infinitely selective membrane, the concentration ofthe preferentially permeating component on the permeate side of themembrane can never be greater than φ times the concentration in thefeed, where φ is feed pressure/permeate pressure. To achieve an adequatepressure ratio with the permeate pressure at atmospheric requires thatthe feed pressure preferably be above about 60 psi. In many cases, thepurge gas withdrawn from the refrigeration cycle will be at a pressuresubstantially above 60 psi. If the feed pressure to the membrane is notsufficiently high to provide a useful pressure ratio, then a pressuredrop across the membrane can be provided by drawing a partial vacuum onthe permeate side of the membrane. Subatmospheric pressure on thepermeate side can also be sustained in some cases simply by continuouslycondensing and withdrawing the permeate stream.

In embodiments using refrigerant-selective membranes, the residue streamwill be the air stream. The refrigerant content of the air should bereduced to a level at which the air can be vented to the atmosphere withminimal loss of refrigerant or environmental pollution. Preferably theresidue stream should contain less than 10%, more preferably less than5% of the refrigerant that was in the feed to the membrane unit. Ifair-selective membranes are used, the permeate will be the air streamand similarly should be clean enough for discharge.

The process of the invention can be carried out using membrane systemdesigns tailored to particular requirements in terms of the compositionof the feed to the membrane unit, and the desired compositions of theresidue and permeate streams. The purge gas stream may optionally besubjected to a condensation step to recover a substantial portion of therefrigerant, followed by the membrane treatment step. Somerepresentative embodiments of the invention are described below. Theseembodiments are illustrative of workable configurations, but are notintended to limit the scope of the invention in any way. Those of skillin the refrigeration or membrane arts will appreciate that many otherembodiments in accordance with the invention are possible.

REPRESENTATIVE EMBODIMENTS USING REFRIGERANT-SELECTIVE MEMBRANE

1. Purge-gas treatment step comprises condensation followed by membraneseparation:

A preferred mode for carrying out the invention is to subject thewithdrawn purge gas to a condensation step that precedes the membranetreatment step. If the purge gas contains a high percentage ofrefrigerant, and is at a high pressure, both of which will usually bethe case, then cooling the gas stream will cause a fraction of therefrigerant process using this treatment scheme is shown in FIG. 1.Referring now to this figure, the refrigeration cycle, 1, is a singlevapor cycle. Compressor, 2, creates a region of high-pressurerefrigerant vapor, 3. The vapor passes into a heat-exchange zone, 4,where heat is given off to a coolant and the vapor condenses to create ahigh-pressure liquid zone, 5. The refrigerant then passes throughexpansion valve, 6, to a low-pressure liquid zone, 7. Heat exchangebetween the product to be refrigerated and the refrigerant takes placein evaporator zone, 8. The resulting low-pressure vapor in zone, 9, isrecompressed and the cycle starts again. A high-pressure purge stream,12, is withdrawn from the refrigeration cycle through outlet, 10, andvalve, 11. The purge stream is passed through condenser, 13, which maybe simply a water or air-cooled condenser operating at above 0° C., ormay be refrigerated, either by making use of the existing refrigerationcycle or by means of a separate, smaller refrigerator. Condensertemperatures down to about -45° C. can be reached in a single-cyclechilling operation. If a lower condenser temperature is used, a compoundor cascade system could be used. This would be a very undesirable modeof operation, because of the complexity and high energy consumption,unless the refrigeration cycle itself were a compound or cascade cycle,through which the purge stream could easily be fed. The presence of themembrane treatment unit means that the amount of refrigerant removed bythe condenser is not a critical factor in the design. The combinedcondensation/membrane separation treatment step may be tailored so thatthe condensation step can be performed above 0° C. This can beadvantageous in situations where the purge gas stream contains watervapor, for example, in embodiments using an absorption refrigerationcycle, because the need to defrost the condenser regularly will then beavoided. On the other hand, the selectivity of some polymers for organicvapors over air increases with decreasing temperature. In cases wherethe refrigerant selectivity at room temperature is poor, therefore, abetter separation may be obtained in the membrane step by chilling thepurge gas to a relatively low temperature before it passes through themembrane unit.

The fraction of refrigerant remaining in the purge gas stream after thecondensation step depends on the vapor/liquid equilibrium at theoperating conditions under which the condensation step is performed. Ifa condensation step is used, it is generally preferable that thecondensation step be designed to remove at least 50% of the refrigerantpresent in the withdrawn purge stream. Operation under extremeconditions, to achieve 95% or more refrigerant removal is usuallyunnecessary, because of the presence of the membrane step. The overalldegree of condensable removal and recovery that can be achieved by thecombined condensation/membrane separation step is a multiple of theremoval achieved in the individual steps. For example, suppose thecondensation step removes 50% of the refrigerant. If the condensationstep is followed by a membrane separation step that can remove 80% ofthe refrigerant reaching it, then the total removal obtained by theprocess is 90%. If the condensation step removes 80%, and is followed bya membrane separation step that also removes 80%, then the total removalobtained by the process is 96%. If the condensation step removes 80% andthe membrane separation step 90%, the total removal is 98%.

The above discussion is intended to show that the process can betailored to achieve a desired degree of refrigerant in a highlyefficient manner. The tailoring can be done by comparing estimates ofthe energy and dollar costs with several sets of system configurationsand operating conditions. For example, the costs and energy requirementsto achieve 95% total removal, using an initial condensation stepremoving 50, 75 or 90% of refrigerant component, followed by a membraneseparation step removing 90, 80 or 50% of the remaining refrigerant,could be compared.

The liquid refrigerant stream, 14, from the condenser may be drawn offfor reuse in the refrigeration cycle. Stream, 15, which is at highpressure compared with atmospheric and which contains non-condensedrefrigerant and air, passes to membrane unit, 16, containing one or moremembranes that are selectively permeable to the refrigerant. Thepermeate stream, 18, is therefore enriched in refrigerant compared withstream 15. If the selectivity is high, and the stream from the condenseris not too dilute, permeate stream, 18, may be sufficiently concentratedto be fed back to the refrigeration cycle. FIGS. 2 and 3 show suchoptions. The residue stream, 17, from the membrane operation, which maystill be at above atmospheric pressure, is sufficiently reduced inrefrigerant concentration that it can be discharged to the atmosphere.Preferably this residue stream contains less than 1% refrigerant, morepreferably less than 0.5% refrigerant.

2. Permeate vapor is returned to refrigeration cycle:

Referring now to FIG. 2, this is a variation of the embodiment ofFIG. 1. In this case, the permeate stream, 19, is withdrawn from themembrane separation operation and returned directly to the low-pressurevapor zone of the refrigeration cycle. This option requires noadditional compressor or condenser to handle the permeate stream. On theother hand, because membranes are not infinitely selective, the permeatestream will always contain some air that will, therefore, reenter therefrigeration cycle, and have to pass through the main compressor again.

3. Permeate vapor is recompressed, then returned to condenser:

Referring now to FIG. 3, permeate stream, 20, is compressed incompressor, 20, and returned to the condenser, 15. This may bepreferable to feeding the permeate vapor directly back to therefrigeration cycle, because compressor, 20, is relatively small, and noair is fed back to the refrigeration cycle. The overall energyconsumption of this system may therefore be less than that of FIG. 2.

REPRESENTATIVE EMBODIMENTS USING AIR-SELECTIVE MEMBRANE

1. Air-selective membrane treatment only:

Other embodiments of the invention involve a membrane treatment step inwhich air-selective membranes are used to directly treat the purge gasstream, without a prior condensation. A basic form of such an embodimentis shown in FIG. 4. Referring now to this figure, the refrigerationcycle and the purge withdrawal operate in the same manner as in theembodiments of FIGS. 1, 2 and 3. The purge stream, 12, at high pressurecompared with atmospheric pressure, is passed directly to membrane unit,22, containing an air-selective membrane. The permeate stream, 24, isthus enriched in nitrogen and oxygen compared with stream 12. Theresidue stream contains concentrated refrigerant. If the purge gasconcentration, membrane selectivity and operating parameters are suchthat a very good separation is achieved in the simple, single-stagemembrane operation, it may be possible to vent the permeate stream andto return the residue stream, with or without liquifying it, to therefrigeration cycle, as in the refrigerant-selective embodiments ofFIGS. 2 and 3. Unless extremely selective membranes, having aselectivity for nitrogen over refrigerant of 200, 500 or even more areavailable, it is likely that the permeate stream, at least, will requirefurther treatment. However, in a membrane separation process, it isusually desirable to keep the stage-cut low to achieve a goodseparation. Thus the volume of the permeate stream is normally muchsmaller than the volume of the feed and residue streams. An advantage ofair-selective embodiments such as FIG. 4, therefore, is that the volumeof the permeate air stream containing refrigerant vapor that must becondensed or otherwise treated is very small compared with the totalpurge gas volume. This means it may become economically viable tooperate the refrigeration cycle with the air content maintained at alower level than was previously possible, thereby reducing the headpressure that must be achieved by the refrigerant cycle compressor.

2. Combination of air-selective and refrigerant-selective membranes:

The purge gas stream as it is withdrawn from the refrigeration cycle maytypically contain 90% refrigerant and 10% air, for example. If the purgestream is passed across a membrane having a selectivity for nitrogenover the refrigerant of about 20, for example, then even if thestage-cut is kept low, the permeate stream may still contain 20-30%refrigerant. This level is far too high to discharge the permeate streamdirectly to the atmosphere. However if the permeate stream, 24, is nowpassed through a refrigerant-selective membrane unit, the refrigerantconcentration can be reduced to a point where discharge is possible. Theresidue stream from the refrigerant-selective membrane operation couldnow be discharged, and the permeate stream from therefrigerant-selective unit could be recycled to the feed side of theair-selective unit.

In embodiments incorporating an air-selective membrane unit followed bya refrigerant-selective membrane unit, it will normally be necessary tolower the pressure on the permeate side of the refrigerant-selectiveunit below atmospheric pressure, because the feed to this unit will beat, or close to, atmospheric pressure.

3. Combination of air-selective membrane and condensation:

a) Purge-gas treatment step comprises membrane treatment followed bycondensation step:

This process is the same as that described in embodiment 1 as far as therefrigeration cycle, the purge gas withdrawal and the passage throughthe air-selective membrane are concerned. In this case, however, thepermeate stream, 24, is optionally recompressed and then cooled,preferably using the cooling portion of the refrigerant cycle, torecover a stream of liquid refrigerant suitable for return to therefrigeration cycle. The non-condensed gases, containing only very smallquantities of refrigerant vapor, may now be discharged.

b) Purge-gas treatment step comprises membrane treatment preceded bycondensation step:

To maximize the advantages of the embodiments of the invention describedabove, using air-selective membranes in the membrane treatment step, itis very desirable to use membranes that are highly selective fornitrogen over the refrigerant vapor.

However, if membranes with selectivities for nitrogen over refrigerantof 10 or 20, for example, are available, it is still possible to designuseful embodiments, such as that shown in FIG. 5. Referring now to thisfigure, the refrigeration cycle and the purge gas withdrawal operationare as described for FIG. 4. In this case, however, purge stream, 12, athigh pressure compared with atmospheric pressure, passes to condenser,13. As in the refrigerant-selective embodiments, this condenser may be awater or air-cooled condenser operating at above 0° C., or may berefrigerated, either by making use of the existing refrigeration cycleor by means of a separate, smaller refrigerator. Condenser temperaturesdown to about -45° C. can be reached in a single-cycle chillingoperation; lower temperatures require a compound or cascade system. Aswith the refrigerant-selective options, it is convenient and cheap touse the existing refrigeration cycle to chill the condenser. Liquidrefrigerant stream, 14, from the condenser is suitable for reuse. Thenon-condensed fraction, 15, from the condenser, which is at highpressure compared with atmospheric, is fed to membrane unit, 16,containing an air-selective membrane. The permeate stream, 28, from themembrane unit is mostly air containing only a very low concentration ofrefrigerant vapor. The residue stream, 29, contains refrigerant vapor atabove atmospheric pressure. FIG. 5 shows an option in which this streamis returned to the high-pressure vapor segment of the refrigerationcycle. A recirculation blower or pump may optionally be used to maintainadequate flow of the recirculating vapor stream. Alternatively stream 29could be liquified and returned to the liquid segment of therefrigeration cycle.

It may be seen from the above discussions that the purge-gas treatmentoperation may be configured in many different ways, tailored to achievea highly efficient and economic recovery of refrigerant, and to minimizethe atmospheric discharge of waste refrigerant vapors. Depending on therefrigerant that is used, the operating conditions of the refrigerationcycle, and the ability to use existing compression and/or condensationequipment, many different workable and practical embodiment could bedesigned. The goal of all modes of operation is that the purge-gastreatment operation produce only two streams: one a vent streamsufficiently free of refrigerant vapor that its discharge to theatmosphere has no adverse environmental effects; the other a productstream containing sufficiently pure refrigerant for return to therefrigeration cycle.

For simplicity, all the refrigeration processes discussed above havebeen described in terms of a simple, single-stage membrane operation. Aswill be appreciated by those of skill in the art, the membraneseparation operation may be configured in many possible ways, and mayinclude a single membrane stage or an array of two or more units inseries or cascade arrangements. For example, a membrane array consistingof a two-stage cascade is shown schematically in FIG. 6. This type ofmembrane configuration could be used, for example, when the purge streamhas been first subjected to condensation, and where the non-condensedgas from the condenser contains the refrigerant in a low concentration,such that a single pass through a membrane unit would not concentratethe refrigerant vapor to make it return to the refrigeration cycledesirable.

Referring now to FIG. 6, incoming purge stream, 30, containingrefrigerant and air, passes to first membrane separation unit, 31, whichcontains membranes selectively permeable to the refrigerant. Thenon-permeating, residue stream, 32, is thus depleted in the refrigerant.The permeate stream, 33, is enriched in refrigerant, but still containssignificant amounts of nitrogen and oxygen. The permeate from the firstmembrane unit, now at atmospheric pressure, is fed to second membraneunit, 34. A pressure difference across the second membrane unit isprovided by vacuum pump, 35. The permeate stream, 36, from the secondmembrane unit is now highly concentrated in refrigerant and can becondensed in condenser, 37, to produce a liquid refrigerant stream, 38.Any non-condensed fraction, 39, can be recycled to the second membraneunit. The residue stream, 40, from the second membrane unit, depleted inrefrigerant compared with stream 33, may optionally be recycled to thefeed side of the first membrane unit. In this way the membrane operationproduces only two streams, the liquid refrigerant stream, 38, suitablefor return to the refrigeration cycle, and the essentially clean residuestream, 32, for discharge.

A second membrane array, consisting of a two-step series arrangement, isshown schematically in FIG. 7. This type of membrane unit could be used,for example, when essentially complete removal of refrigerant beforeventing is required. A two-step process will typically remove 99% ormore of the refrigerant reaching it. Referring now to FIG. 7, incomingpurge gas stream, 42, containing refrigerant and air, is passed to firstmembrane separation unit, 43, which contains membranes selectivelypermeable to the refrigerant. The permeate stream, 49, is enriched inthe refrigerant and can be optionally liquified by means of compressor,50, and condenser, 51, to yield a liquid refrigerant stream, 52,suitable for return to the refrigeration cycle. The residue stream, 44,is depleted in refrigerant compared with stream 42, but still containstoo much refrigerant for discharge. Stream 44 is therefore fed to secondmembrane unit, 45. The residue stream, 46, from the second membrane unitis now sufficiently clean for discharge. The permeate stream, 48, fromthe second membrane unit, enriched in refrigerant compared with stream42, may be recompressed by compressor, 47, and recycled to the feed sideof the first membrane unit. In this way the membrane operation producesonly two streams, the liquid refrigerant stream, 52, and the relativelyclean air stream, 46.

Multiple-stage and multiple-step membrane operations, and combinationsof these, could be used with embodiment using refrigerant-selective orair-selective membranes.

The invention is now further illustrated by the following examples,which are intended to be illustrative of the invention, but are notintended to limit the scope or underlying principles of the invention inany way. The example are divided into two groups. The first group,Examples 1-13 shows the separation performance that can be achieved fora number of common refrigerants using a typical thin-film compositemembrane. The second group, Examples 14-27, shows typical performancesachieved in the purge stream treatment operation.

EXAMPLES Group 1 Examples

Examples 1-11. Experimental results with refrigerant-selectivemembranes.

Experimental Procedure

All sample feedstreams were evaluated in a laboratory test systemcontaining one membrane module with a permselective silicone rubbermembrane and membrane area of approximately 2,000 cm². The air in thefeed cycle was replaced with nitrogen from a pressure cylinder prior tothe experiment. Nitrogen was continuously fed into the system during theexperiment to replace the lost nitrogen into the permeate. Solvent vaporwas continuously fed into the system by either pumping liquid solventinto the residue line using a syringe pump and evaporating the solventusing additional heating, or sending a bypass stream of the residuethrough a wash bottle containing the liquid solvent. The feed andresidue organic concentrations were determined by withdrawing samplesfrom the appropriate lines by syringe and then subjecting these to gaschromatograph (GC) analysis. A small bypass stream was used to take thesamples at atmospheric pressure instead of the elevated pressure in thelines. Two liquid nitrogen traps were used to condense the solventcontained in the permeate stream. For long-term experiments, anon-lubricated rotary-vane vacuum pump was used on the permeate side ofthe module. The samples from the permeate stream were taken using adetachable glass vessel constantly purged with a bypass stream of thepermeate. Upon sampling, the vessel was detached and air was allowed toenter the vessel. The concentration in the vessel was determined by gaschromatography. The permeate concentration was then calculated from therelationship: ##EQU1##

The procedure for a test with the system was as follows:

1. The system was run without solvent under maximum permeate vacuum toreplace the air in the loop with nitrogen.

2. The nitrogen permeate flow rate was determined by measuring thevacuum pump exhaust flow rate. This provided a quality check on themodule.

3. The feed flow, feed pressure and permeate pressure were adjusted tothe desired values. The cold trap was filled with liquid nitrogen.

4. The solvent input was started and the feed concentration wasmonitored with frequent injections into the GC. The permeate pressurewas adjusted if necessary.

5. The system was run until the feed analysis showed that steady statehad been reached.

6. All parameters were recorded and a permeate sample was taken andanalyzed.

7. Step 6 was repeated after 10-20 minutes. The feed concentration wasmonitored after each parameter change to ensure steady state had beenreached.

EXAMPLE 1 CFC-11. Low Concentrations

The experimental procedures described were carried out using afeedstream containing CFC-11 (CCl₃ F) in concentrations from 100-2,000ppm. The results are summarized in FIGS. 8 and 9. The calculated CFC/N₂selectivity of the module increased slightly from 22 at 100 ppm to 28 at2,000 ppm. As can be seen from FIG. 6, up to about 4 lb/m².day of CFC-11could be recovered, even from a very dilute stream in a very simpleone-step process.

EXAMPLE 2 CFC-11. Higher Concentrations

The experimental procedures described were carried out using afeedstream containing CFC-11 (CCl₃ F) in concentrations from 1-35 vol %.The results are summarized in FIGS. 10 and 11. The calculated CFC/N₂selectivity of the module increased from 30 at 1 vol % to 50 at 35 vol%. This effect may be attributable to plasticization of the membranematerial by sorbed hydrocarbon. Both hydrocarbon and nitrogen fluxesincreased with increasing hydrocarbon feed concentration.

EXAMPLE 3 CFC-113. Low Concentrations

The experimental procedures described were carried out using afeedstream containing CFC-113 (C₂ Cl₃ F₃) in concentrations from500-2,000 ppm. The results are summarized in FIG. 12. The calculatedCFC/N₂ selectivity of the module remained constant at about 20 over thefeed concentration range.

EXAMPLE 4 CFC-113. Higher Concentrations

The experimental procedures described were carried out using afeedstream containing CFC-113 (C₂ Cl₃ F₃) in concentrations from 0.5-6vol %. The results are summarized in FIG. 13. The calculated CFC/N₂selectivity of the module remained constant at about 25 over the feedconcentration range.

EXAMPLE 5 HCFC-123. Low Concentrations

The experimental procedures described were carried out using afeedstream containing HCFC-123 (C₂ HCl₂ F₃) in concentrations from500-2,000 ppm. The results are summarized in FIG. 14. The calculatedCFC/N₂ selectivity of the module remained constant at about 25 over thefeed concentration range.

EXAMPLE 6 HCFC-123. Higher Concentrations

The experimental procedures described were carried out using afeedstream containing HCFC-123 (C₂ HCl₂ F₃) in concentrations from 0.5-8vol %. The results are summarized in FIG. 15. The calculated CFC/N₂selectivity of the module remained constant at about 25 over the feedconcentration range.

EXAMPLE 7 HCFC-142b.

The experimental procedures described were carried out using afeedstream containing HCFC-142b (C₂ H₃ ClF₂) in concentrations from300-3,500 ppm. The results are summarized in FIG. 16. The calculatedCFC/N₂ selectivity of the module increased very slightly from 13 to 15over the feed concentration range.

EXAMPLE 8 CFC-114

The experimental procedures described were carried out using afeedstream containing CFC-114 (C₂ Cl₂ F₄) in concentrations from 2-25vol %. The results are summarized in FIG. 17. The calculated CFC/N₂selectivity of the module increased very slightly from about 9 to 12over the feed concentration range.

EXAMPLE 9 Halon-1301

The experimental procedures described were carried out using afeedstream containing Halon-1301 (CF₃ Br) in concentrations from 0.1-5vol %. A Halon/nitrogen selectivity of about 4 was obtained.

EXAMPLE 10 Carbon Dioxide

Membranes particularly suited to the separation of carbon dioxide,sulfur dioxide and ammonia from air can be prepared from commerciallyavailable poly(ether amide ester) block copolymers. A thin-filmcomposite membrane was prepared by coating a 1-3% solution of Pebax®4011 (Atochem, Inc.) in butanol onto a polysulfone microporous supportmembrane. Permeability experiments were carried out using a feed streamcontaining 8% carbon dioxide in an air mixture. The carbon dioxide fluxat 61° C. was 6.05×10⁻⁴ cm³ (STP)/cm² ·s·cmHg. The calculated carbondioxide/nitrogen selectivity was 26.

EXAMPLE 11 Sulfur Dioxide

A thin-film composite membrane was prepared as in Example 10.Permeability experiments were carried out using a feed stream containing0.33% sulfur dioxide in an air mixture. The sulfur dioxide flux at 61°C. was 6.12×10⁻³ cm³ (STP)/cm² ·s·cmHg. The calculated sulfurdioxide/nitrogen selectivity was 251.

EXAMPLE 12 Air-selective Membranes

Asymmetric Loeb-Sourirajan membranes were prepared using a castingsolution of 14.4 wt % polyethersulfone (Victrex 52009ICI Americas)dissolved in 47.9% methylene chloride, 24% 1,1,2-trichloroethane, 6%formic acid and 7.7% butanol. The casting solution was spread on a glassplate using a hand-held spreader roll. The glass plate was then immersedin a methanol bath, causing the polymer to precipitate. After theprecipitation was complete, the membranes were removed and dried. Themembranes were overcoated with a 0.5- to 2-μm-thick layer of siliconerubber dissolved in octane. This silicone rubber layer sealed themembrane defects and the permselectivity of the membrane was then closeto the intrinsic values obtained with thick isotropic films of thepolymer.

Permeation experiments were carried out as above, using various dilutemixtures of CFC-11 in air. The membranes exhibited an oxygen flux of2.85×10⁻⁶ cm³ (STP)/cm² ·s·cmHg, a nitrogen flux of 7.45×10⁻⁷ cm³(STP)/cm² ·s·cmHg and a CFC-11 flux of 4.75×10⁻⁸ cm³ (STP)/cm² ·s·cmHg.The nitrogen/CFC-11 selectivity was 16.

EXAMPLE 13 Air-selective Membranes

Asymmetric Loeb-Sourirajan membranes were prepared by a similartechnique to that described in Example 12, but using a casting solutionof 10 wt % polyphenylene oxide dissolved in 85% 1,1,2-trichloroethyleneand 5% octanol. The casting solution was spread on a glass plate using ahand-held spreader roll. The glass plate was then immersed in a methanolbath, causing the polymer to precipitate. After the precipitation wascomplete, the membranes were removed and dried. The membranes wereovercoated with a 0.5- to 2-μm-thick layer of silicone rubber dissolvedin octane. This silicone rubber layer sealed the membrane defects andthe permselectivity of the membrane was then close to the intrinsicvalues obtained with thick isotropic films of the polymer.

Permeation experiments were carried out as above, using various dilutemixtures of CFC-11 in air. The membranes exhibited an oxygen flux of1.20×10⁻⁶ cm³ (STP)/cm² ·s·cmHg, a nitrogen flux of 2.90×10⁻⁷ cm³(STP)/cm² ·s·cmHg and a CFC-11 flux of 3.68×10⁻⁹ cm³ (STP)/cm² ·s·cmHg.The nitrogen/CFC-11 selectivity was 79.

GROUP 2 EXAMPLES EXAMPLES 14-18 Design and Analysis of DifferentPurge-Gas Treatment Operations

This set of examples compares treatment of a CFC-11 laden stream bycondensation alone and by the purge-gas treatment operation of theinvention. Examples 14-16 are not in accordance with the invention. Thestream has a flow rate of 10 scfm and contains 50% CFC-11 in all cases.The membrane calculations are all based on CFC-11 selectivitiesdetermined in single module experiments of the type described in thefirst group of examples. The calculations were performed using acomputer program based on the gas permeation equations for cross flowconditions described by Shindo et al., "Calculation Methods forMulticomponent Gas Separation by Permeation," Sep. Sci. Technol. 20,445-459 (1985). The membrane area required was generated by the computerprogram. The chiller capacity was extrapolated from product literatureprovided by Filtrine Manufacturing Company, of Harrisville, N.H. Thecapacities of the vacuum pumps and compressors were obtained orextrapolated from performance specification charts and other data fromthe manufacturers. Energy calculations were done by calculating theadiabatic ideal work of compression and dividing by the efficiency ofthe unit. Compressor efficiency was taken to be 60%: vacuum pumpefficiency was taken to be 35%.

EXAMPLE 14 Compression to 5 Atmospheres Plus Chilling to 7° C.

The CFC-11 laden purge stream is considered to be at a pressure of 5atmospheres, and is chilled to 7° C. and condensed. The performance ischaracterized as shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        Stream      Composition    Flow rate                                          ______________________________________                                        Feed        50% CFC-11 in air                                                                            10 scfm                                            Liquid condensate                                                                         Pure CFC-11    0.77 kg/min                                        Non-condensed off-                                                                        10.9% CFC-11   5.6 scfm                                           gas from condenser:                                                           Fractional recovery of CFC from feed: 88%                                     Energy requirement (hp)                                                       Total: 2.96 Compressor: 1.96                                                                             Chiller/condenser: 1                               ______________________________________                                    

EXAMPLE 15 Compression to 25 Atmospheres Plus Chilling to 7° C.

The CFC-11 laden purge stream is compressed to 25 atmospheres, thenchilled to 7° C. and condensed. The performance is characterized asshown in Table 3.

                  TABLE 3                                                         ______________________________________                                        Stream      Composition   Flow rate                                           ______________________________________                                        Feed        50% CFC-11 in air                                                                           10 scfm                                             Liquid condensate                                                                         Pure CFC-11   0.86 kg/min                                         Non-condensed off-                                                                        2.18% CFC-11  5.11 scfm                                           gas from condenser:                                                           Fractional recovery of CFC from feed: 98%                                     Energy requirement (hp)                                                       Total: 6.14 Compressor: 5.04                                                                            Chiller/condenser: 1.1                              ______________________________________                                    

EXAMPLE 16 Compression to 5 Atmospheres Plus Chilling to -27° C.

This example achieves the same performance as Example 15 above, bytaking the purge gas at 5 atm pressure, but using a lower chillertemperature of -27° C. The performance is characterized as shown inTable 4.

                  TABLE 4                                                         ______________________________________                                        Stream      Composition   Flow rate                                           ______________________________________                                        Feed        50% CFC-11 in air                                                                           10 scfm                                             Liquid condensate                                                                         Pure CFC-11   0.86 kg/min                                         Non-condensed off-                                                                        2.18% CFC-11  5.11 scfm                                           gas from condenser:                                                           Fractional recovery of CFC from feed: 98%                                     Energy requirement (hp)                                                       Total: 7.46 Compressor: 1.96                                                                            Chiller/condenser: 5.5                              ______________________________________                                    

EXAMPLE 17 Purge-Gas Treatment Operation In Accordance With AnEmbodiment of The Invention

A process was designed to achieve the same level of performance as inExamples 15 and 16. The process involved a condensation step followed bya membrane separation step. In the condensation step, the CFC-11 ladenstream, at 5 atmospheres pressure, is chilled to 7° C. and condensed.The non-condensed off-gas from the condensation step is then subjectedto a membrane separation step, using a membrane with a selectivity forCFC-11 over air of 30. A pressure drop across the membrane is providedonly by the elevated pressure of the compressed feed. The permeatestream from the membrane separation step is returned for treatment inthe condensation step. The performance is characterized as shown inTable 5.

                  TABLE 5                                                         ______________________________________                                        CONDENSATION STEP:                                                            Stream     Composition    Flow rate                                           ______________________________________                                        Feed       50% CFC-11 in air                                                                            10 scfm input + 3.33                                           input + 24.3% from                                                                           scfm returned from                                             membrane = 43.6%                                                                             membrane step =                                                               13.33 scfm                                          Liquid condensate                                                                        Pure CFC-11    0.86 kg/min                                         Condenser off-gas                                                                        10.9% CFC-11   8.44 scfm                                           ______________________________________                                        MEMBRANE SEPARATION STEP:                                                     Stream     Composition    Flow rate                                           ______________________________________                                        Feed       10.9% CFC-11   8.44 scfm                                           Residue    2.18% CFC-11   5.11 scfm                                           Permeate   24.3% CFC-11   3.33 scfm                                           Membrane area: 4.17 m.sup.2                                                   Stage cut: 40%                                                                Fractional recovery of CFC from feed: 98%                                     Energy requirement (hp)                                                       Total: 3.91                                                                              Compressor: 2.61                                                                             Chiller/condenser: 1.3                              ______________________________________                                    

Comparing this example with Examples 15 and 16, it may be seen that theprocess of the invention can reduce the energy demands for a treatmentsystem to remove and recover 98% of the CFC from either 7.46 hp or 6.14hp to 3.91 hp. In other words, the energy usage of the process is only52% or 64% that of the comparable condensation process alone.

EXAMPLE 18 Purge-Gas Treatment Operation Employing the Process of theInvention

The process as in Example 17 was again considered. The only differencewas the inclusion of a small vacuum pump on the permeate side of themembrane to lower the permeate pressure to 15 cmHg. The performance ischaracterized as shown in Table 6.

                  TABLE 6                                                         ______________________________________                                        CONDENSATION STEP:                                                            Stream     Composition    Flow rate                                           ______________________________________                                        Feed       50% CFC-11 in air                                                                            10 scfm input + 1.16                                           input + 49.3% from                                                                           scfm returned from                                             membrane = 49.9%                                                                             membrane step =                                                               11.16 scfm                                          Liquid condensate                                                                        Pure CFC-11    0.86 kg/min                                         Condenser off-gas                                                                        10.9% CFC-11   6.27 scfm                                           ______________________________________                                        MEMBRANE SEPARATION STEP:                                                     Stream     Composition    Flow rate                                           ______________________________________                                        Feed       10.9% CFC-11   6.27 scfm                                           Residue    2.18% CFC-11   5.11 scfm                                           Permeate   49.3% CFC-11   1.16 scfm                                           Membrane area: 0.81 m.sup.2                                                   Stage cut: 18%                                                                Fractional recovery of CFC from feed: 98%                                     Energy requirement (hp)                                                       Total: 3.93                                                                              Compressor: 2.19                                                                             Chiller/condenser: 1.3                                         Vacuum pump 0.44                                                   ______________________________________                                    

Comparing this example with Example 17, several differences areapparent. To reduce the residue concentration to 2.18% in Example 17requires a relatively high stage cut of 40%. The permeate volume flow ishigh, 3.33 scfm, so a more powerful compressor is needed to handle theadditional load returned from the membrane unit. The membrane area, 4.17m², is also large. The use of a vacuum pump to lower the pressure on thepermeate side means that the same degree of CFC removal can be achievedwith a much smaller membrane area, 0.81 m², and a much lower stage cut,18%. There is a corresponding saving in the energy requirements of thecompressor. However, the energy used by the vacuum pump makes theoverall energy demand of the system about the same in both cases. Bothschemes achieve major improvements in performance compared withcondensation alone.

EXAMPLES 19-21

This set of examples compares treatment of a gas stream containingsulfur dioxide in air by condensation alone and by a representativepurge-gas treatment operation in accordance with the invention. Thestream has a flow rate of 10 scfm and contains 50% sulfur dioxide in allcases. The calculations are performed in similar manner to those for theCFC-11 examples. The membrane calculations were based on the performanceof composite membranes having a permselective layer ofpolyamide-polyether block copolymer. The membrane selectivity for sulfurdioxide over air was taken to be 100, and the normalized sulfur dioxideflux was 6×10⁻³ cm³ (STP)/cm² ·s·cmHg.

EXAMPLE 19 Compression to 8 Atmospheres Plus Chilling to 6° C.

The sulfur dioxide laden purge stream is considered to be available at 8atmospheres pressure, and is chilled to 6° C. and condensed. The boilingpoint of sulfur dioxide is -10° C., so under these conditions 25% sulfurdioxide remains in the vent gas from the condenser. The performance ischaracterized as shown in Table 7.

                  TABLE 7                                                         ______________________________________                                        Stream           Composition  Flow rate                                       ______________________________________                                        Feed             50% SO.sub.2 in air                                                                        10 scfm                                         Liquid condensate                                                                              Pure SO.sub.2                                                                              0.3 kg/min                                      Non-condensed    25% SO.sub.2 6.25 scfm                                       off-gas from condenser:                                                       ______________________________________                                    

EXAMPLE 20 Compression to 40 Atmospheres Plus Chilling to 6° C.

The sulfur dioxide laden stream is compressed to 40 atmospheres, thenchilled to 6° C. and condensed. The sulfur dioxide content of the ventgas is reduced to 5% under these conditions, but the energy and costrequirements of the system are more than double those of Example 19. Theperformance is characterized as shown in Table 8.

                  TABLE 8                                                         ______________________________________                                        Stream           Composition  Flow rate                                       ______________________________________                                        Feed             50% SO.sub.2 in air                                                                        10 scfm                                         Liquid condensate                                                                              Pure SO.sub.2                                                                              0.38 kg/min                                     Non-condensed    5% SO.sub.2  5.26 scfm                                       off-gas from condenser:                                                       ______________________________________                                    

EXAMPLE 21 Purge-Gas Treatment Operation in Accordance With theInvention

A process was designed employing the condensation step exactly as inExample 19, followed by a membrane separation step to treat thecondensation step vent gas stream, using a membrane with a selectivityfor sulfur dioxide over air of 100. A pressure drop across the membraneis provided only by the elevated pressure of the compressed feed. Theperformance is characterized as shown in Table 9.

                  TABLE 9                                                         ______________________________________                                        Stream           Composition  Flow rate                                       ______________________________________                                        CONDENSATION STEP                                                             Feed             50% SO.sub.2 in air                                                                        10 scfm                                         Liquid condensate                                                                              Pure SO.sub.2                                                                              0.40 kg/min                                     Non-condensed    25% SO.sub.2 6.25 scfm                                       off-gas from condenser:                                                       MEMBRANE SEPARATION STEP:                                                     Feed             25% SO.sub.2 6.25 scfm                                       Residue          1.0% SO.sub.2                                                                              5.05 scfm                                       Permeate         55.6 SO.sub.2                                                                              1.20 scfm                                       ______________________________________                                    

The permeate from the membrane separation step is richer in sulfurdioxide content than the original gas stream to be treated, and can bereturned for treatment by the condensation step. The process is able toreduce the concentration of sulfur dioxide in the vented gas stream from25% to 1%, with no extra energy consumption whatsoever, because thedriving force for membrane permeation is provided by the relatively highpressure of the already compressed feed.

EXAMPLE 22 Propylene/Ethylene Cascade Refrigeration Cycle

A two-stage cascade refrigeration cycle employing ethylene and propyleneis used to produce refrigeration at -145° F. A system of this type isdescribed in FIG. 8.26, page 226 in Chemical Process Equipment Handbook,Butterworth's Series in Chemical Engineering. In this system, propylenevapor in the first stage is compressed to 245-250 psia and cooled withwater to 116° F. forming the liquid propylene. This liquid is thenexpanded to 16 psia to produce a cold vapor. This cold vapor is passedthrough a heat exchanger and provides cooling to liquify ethylene vaporat a pressure of 230-240 psia in the second stage of the cascade.Expansion of this ethylene liquid to 12 psia produces an ethylene vaporat a temperature of -145° F. The low-pressure portion of the ethylenecycle is subject to air leaks. Suppose that the purge stream from theethylene cycle contains 2-10% air. Typically, this purge stream willfirst be cooled to -140° F. in a purge-gas condenser. At a purge gaspressure of 240 psia, the condensation operation will produce a liquidethylene stream and a non-condensed stream, consisting of 90% air and10% ethylene, at a rate of approximately 10 scfm. This pressurized ventgas is most economically passed across the surface of a silicone rubbermembrane. This membrane is 8 times more permeable to ethylene thannitrogen and 4 time more permeable to ethylene than oxygen. The membranethus fractionates the gas into a 5.2 scfm residue stream containing 99%air and 1% ethylene, which can be discharged to the atmosphere, and alow-pressure permeate stream containing 19.7% ethylene and 80.3% air.The permeate stream may be returned directly to the low-pressure side ofthe refrigeration cycle, or recompressed to 240 psia and introduced infront of the -140° F. purge-gas condenser.

EXAMPLE 23 Ammonia Refrigeration Cycle

Ammonia is often used as a refrigerant in compression refrigerationsystems to provide cooling in the 20° to -50° F. range. In thesesystems, non-condensable gases collect in high pressure side of thecycle and must be removed as a purge stream. Consider, for example, arefrigerator using ammonia with a condenser liquid ammonia temperatureon the high-pressure side of 95° F. At this temperature the vaporpressure of ammonia is 197 psia. However, not uncommonly, the actualoperating pressure will be on the order of 210 psia. The extra 13 psiarepresents 6% non-condensable gases (air, hydrogen, nitrogen, etc.).This gas must be purged at a rate determined by the rate of appearanceof non-condensable gas in the refrigeration cycle. Suppose that thepurge-gas stream, containing 6% air or other non-condensable gases, isfirst subjected to a condensation step using cooling to -40° C. providedby the refrigeration cycle. The ammonia concentration in the vent gasleaving the condenser will be 4.9%. The condenser vent gas is thenpassed to a membrane separation unit, containing a thin-film compositemembrane with a silicone rubber permselective layer. Such a membrane hasa selectivity of 20 for ammonia over nitrogen and 10 for ammonia overoxygen. Depending on the stage-cut, the membrane operation could producea residue stream containing 0.5% ammonia, and a permeate streamcontaining 16% ammonia, down to a residue stream containing about 0.05%ammonia, and a permeate stream containing 10% ammonia. By using atwo-step process, the concentration of ammonia in the residue streamcould be reduced even further if necessary.

EXAMPLE 24 CFC-12 Recovery

Consider an embodiment of the invention as shown in FIG. 1, with CFC-12as refrigerant. Based on our experimental data, we assume a membraneselectivity for CFC-12 over air of 6-10. Consider a purge gas streamcontaining 10 scfm air, contaminated with 67 scfm of CFC-12, to producean 87% CFC-12 stream. The stream emerges from the purge withdrawaloperation at 90 psia, and is first passed through a condenser operatingat -60° F. The condenser reduces the CFC content of the gas to 5%CFC-12. This resulting stream is passed to a membrane unit, whichselectively permeates the CFC. As a result, a 10 scfm stream containing0.5% CFC-12 is formed and can be vented. The permeate stream from themembrane separation operation contains 11% CFC-12, and could berecompressed with a small compressor and passed back to the coldcondenser.

As an alternative, the CFC-12 content of the vent stream could bereduced to 0.05% by using a two-step membrane separation operation asshown in FIG. 7. In this case the permeate from the second step could bereturned to the inlet of the first step. The purge stream treatmentoperation still only produces two streams therefore: the vent gas streamcontaining 0.05% CFC-12, and the liquid CFC-12 stream from thecondensation step.

EXAMPLE 25 Purge-Gas Treatment Using Air-Selective Membrane Step

Consider an embodiment of the invention as shown in FIG. 4, used totreat a purge-gas stream at a pressure of 100 psig, containing 90% of anunspecified refrigerant, mixed with 9.4% nitrogen and 0.6% oxygen.Assume that the membrane selectivity for oxygen over nitrogen is 4, andthat the membrane unit is run with a stage-cut of 1%. Assume furtherthat, to increase the pressure difference across the membrane, a vacuumpump is used on the permeate side of the membrane to lower the pressureon the permeate side to 1 cmHg. Using a computer model based on thecomputational methods of Shindo et al. (Shindo et al., "CalculationMethods for Multicomponent Gas Separation by Permeation," Sep. Sci.Technol. 20, 445-459 (1985)), the amount of refrigerant remaining in thepermeate stream from the membrane unit was calculated as a function ofthe membrane selectivity. The results are summarized in Table 10.

                  TABLE 10                                                        ______________________________________                                        Refrigerant in vent stream as a function of membrane selectivity              for air-selective membranes used to treat refrigerator purge gas.             Selectivity   Refrigerant in exhaust                                          N.sub.2 /refrigerant                                                                        (%)                                                             ______________________________________                                         10           44.4                                                             20           28.9                                                             50           14.2                                                            100           7.7                                                             200           4.0                                                             500           1.64                                                            1,000         0.83                                                            ______________________________________                                    

It may be seen that extremely air-selective membranes are required toreduce the refrigerant content of the purge gas to an acceptable levelin a single pass.

EXAMPLE 26 Purge-Gas Treatment Using Air-Selective Membrane Step andCondensation Step

Consider an embodiment of the invention as shown in FIG. 5, in which acondensation step is used to treat the purge-gas stream prior to themembrane separation step. Suppose that as a result, the refrigerantcontent of the stream being passed to the membrane separation operationis reduced to 5%. Suppose that the feed to the membrane separation stepremains at 100 psig, and that a vacuum pump is used as before to lowerthe permeate pressure to 1 cmHg. The calculation as in Example 22 wasrepeated, and the results are summarized in Table 11.

                  TABLE 11                                                        ______________________________________                                        Refrigerant in vent stream as a function of membrane selectivity              for air-selective membranes used to treat refrigerator purge gas.             Selectivity   Refrigerant in exhaust                                          N.sub.2 /refrigerant                                                                        (%)                                                             ______________________________________                                         10           0.45                                                             20           0.23                                                             50           0.090                                                           100           0.045                                                           200           0.0225                                                          500           0.0090                                                          1,000         0.0045                                                          ______________________________________                                    

As can be seen from the Table, the residue of refrigerant remaining inthe vent gas is now reduced to an extremely low level, even whenmembranes with modest selectivities are used.

EXAMPLE 27 Purge-Gas Treatment Using Air-Selective Membrane Step andCondensation Step

The calculation of Example 26 was repeated, the only difference beingthat no vacuum pump was used, so that the pressure on the permeate sideof the membrane remained at 76 cmHg. The results are summarized in Table12.

                  TABLE 12                                                        ______________________________________                                        Refrigerant in vent stream as a function of membrane selectivity              for air-selective membranes used to treat refrigerator purge gas.             Selectivity   Refrigerant in exhaust                                          N.sub.2 /refrigerant                                                                        (%)                                                             ______________________________________                                         10           0.53                                                             20           0.27                                                             50           0.11                                                            100           0.054                                                           200           0.027                                                           500           0.011                                                           1,000         0.0054                                                          ______________________________________                                    

As can be seen from the Table, the residue of refrigerant remaining inthe vent gas is reduced to an extremely low level, even when membraneswith modest selectivities are used, and a smaller pressure drop isavailable.

We claim:
 1. A process, comprising:a) a refrigeration step, comprisingcompressing and expanding a refrigerant, said step being carried out ina refrigeration system; b) a purge step, comprising withdrawing fromsaid refrigeration system a purge gas comprising said refrigerant andair; c) a purge-gas treatment step, comprising (i) a condensation step,followed by (ii) a membrane separation step.wherein said condensationstep comprises; bringing said purge gas to a condition characterized inthat the concentration of said refrigerant is greater than itssaturation concentration at said condition, so that condensation of aportion of said refrigerant occurs; withdrawing a condensed streamcomprising said refrigerant in liquid form; withdrawing a non-condensedstream depleted in said refrigerant compared with said purge gas; andwherein said membrane separation step comprises; providing a membranehaving a feed side and a permeate side; passing said non-condensedstream from said condensation step across said feed side; withdrawingfrom said permeate side a permeate stream enriched in said refrigerantcompared with said non-condensed stream.
 2. The process of claim 1,wherein said permeate stream is recycled to said refrigeration step. 3.The process of claim 1, wherein said membrane is a composite membranecomprising a microporous support layer and a thin permselective coatinglayer.
 4. The process of claim 1, wherein said membrane has aselectivity for said refrigerant over nitrogen of at least
 5. 5. Theprocess of claim 1, wherein said membrane has a selectivity for saidrefrigerant over air of at least
 10. 6. The process of claim 1, whereinsaid refrigerant comprises a refrigerant selected from the groupconsisting of chlorinated hydrocarbons, CFCs and HCFCs.
 7. The processof claim 1, wherein said refrigerant comprises a refrigerant selectedfrom the group consisting of sulfur dioxide and ammonia.
 8. The processof claim 1, wherein said condensation step comprises chilling said purgegas.
 9. The process of claim 1, wherein said membrane separation step isperformed at a feed gas temperature below ambient temperature.
 10. Theprocess of claim 9, wherein said feed gas is chilled to a temperaturebelow ambient temperature by bringing said feed gas into heattransferring relationship with said refrigeration step before said feedgas is passed across said membrane.
 11. The process of claim 1, whereinat least 90% of said refrigerant present in said feed gas to saidmembrane separation step is recovered in said permeate stream.
 12. Theprocess of claim 1, wherein said permeate stream is recycled to saidcondensation step (i).
 13. The process of claim 1, wherein said membraneseparation step comprises:providing a membrane array, each membranewithin said array having a feed side and a permeate side; passing a feedgas comprising said refrigerant and air across said membrane array;withdrawing from said membrane array a product permeate stream enrichedin said refrigerant compared with said feed gas.
 14. The process ofclaim 13, wherein said membrane array comprises a multiplicity ofmembrane units connected in a series arrangement.
 15. The process ofclaim 13, wherein said membrane array comprises a multiplicity ofmembrane units connected in a cascade arrangement.
 16. The process ofclaim 1, wherein said membrane separation step is performed with saidfeed gas at a pressure of at least 60 psia.
 17. The process of claim 1,wherein said membrane separation step is performed with said feed gas atatmospheric pressure, and wherein a refrigerant flux through themembrane is induced by creating a partial vacuum on said permeate side.18. A process, comprising:a) a refrigeration step, comprisingcompressing and expanding a refrigerant, said step being carried out ina refrigeration system; b) a purge step, comprising withdrawing fromsaid refrigeration system a purge gas comprising said refrigerant andair; c) a purge-gas treatment step, comprising submitting said purge gasto a membrane separation step, wherein membrane separation stepcomprises: passing a feed gas comprising said refrigerant and air acrossa membrane having a feed side and a permeate side; and withdrawing fromsaid feed side a residue stream enriched in said refrigerant comparedwith said feed gas.
 19. The process of claim 18, wherein said purge-gastreatment step comprises (i) a condensation step, followed by (ii) amembrane separation step, and wherein said condensation stepcomprises:bringing said purge gas to a condition characterized in thatthe concentration of said refrigerant is greater than its saturationconcentration at said condition, so that condensation of a portion ofsaid refrigerant occurs; withdrawing a condensed stream comprising saidrefrigerant in liquid form; withdrawing a non-condensed stream depletedin said refrigerant compared with said purge gas; and wherein saidmembrane separation step comprises: providing a membrane having a feedside and a permeate side; passing said non-condensed stream from saidcondensation step across said feed side; withdrawing from said feed sidea residue stream enriched in said refrigerant compared with saidnon-condensed stream.